1. Field of the Invention
The present invention relates to an image sensor such as a proximity image sensor which performs photoelectric conversion while almost contacting an original including image data.
2. Description of the Prior Art
An apparatus having an image data reading function such as a facsimile system or scanner employs a solid-state image sensor for reading image data. Solid-state image sensors used for reading image data include a CCD image sensor and a MOS image sensor. These sensors are used as reduction type image sensors which receive a reduced original image through an optical system, and photoelectrically convert the received image. An apparatus such as a facsimile system assembled with the reduction type image sensor is advantageous in terms of cost. However, such an apparatus has a size limitation since it requires an optical system and a given optical path length.
For meeting the requirement of reducing the apparatus in size, a proximity image sensor which does not require an optical system and a large optical path length, has a length equal to a reading width of an original, and almost contacts an original to read and photoelectrically convert image data has been developed and is commercially available (e.g., Journal of Society of Image Electronics, Vol. 15, pp. 17-26 issued in January, 1986, and Journal of Society of Television, Vol. No. 6, pp. 512-519 issued in June, 1984).
Proximity image sensors can be classified into several types in view of materials, photoelectric conversion/scanning mode, optical systems, and the like. Among these sensors, an accumulation type matrix-drive amorphous silicon image sensor has been receiving a lot of attention. This image sensor employs an amorphous silicon thin film which has good workability, whose nature as a semiconductor film is easily controlled, and which can have a large-area, elongated structure. The image sensor requires smaller numbers of drive ICs and connection wirings with respect to the number of pixels.
This matrix-drive image sensor has such structure that a plurality of photodiodes which are adapted to receive and photoelectrically convert light corresponding in amount to image data for each pixel is aligned in an array, and a blocking diode is arranged opposite to each photodiode and some pairs of photodiodes and blocking diodes are connected in matrix configuration as one unit or block. The blocking diodes are provided to prevent crosstalk between pixels of different blocks which are wired in common and to hold the photodiodes in a reverse bias state, and to separate them from other pixels during an accumulation period. FIG. 1 shows an example of a photoelectric conversion section of a known matrix-drive image sensor.
In the photoelectric conversion section of the matrix-drive image sensor shown in FIG. 1, a lower Cr electrode 2 is formed on a glass substrate 1. A photodiode 3 and a blocking diode 4 which have a pin structure and are formed of an amorphous silicon thin film are formed on the lower electrode 2 to be separated at a distance. An indium-tin-oxide (ITO) transparent conductive film 5 is formed on each of the diodes 3 and 4. Both the diodes are covered with an insulating layer 6 except for the portions of the transparent conductive films 5. Upper electrodes 7 are formed on the insulating layer 6 so that one end of each electrode 7 is connected to the corresponding diode. Diode pairs thus formed are aligned in an array in units of pixels.
FIG. 2 is a plan view showing the matrix-drive image sensor having the array structure for some pixels (three pixels in FIG. 2).
As can be seen from FIG. 1, in the conventional image sensor, the lower electrode 2, the pin photodiode 3, the pin blocking diode 4, the transparent conductive film 5, the insulating film 6, and the upper electrodes 7 are formed in different photoresist steps using different masks. Therefore, the number of steps is increased, resulting in high cost.